In the northern U.S. and Canada when the temperature dips down into the freezing range, fish and plants can be left outdoors to hibernate naturally. The pond should have a recommended depth of at least 2 feet. You should try to clean out as much debris as possible before the pond ices over and install either an aerator, a de-icer, or a heater to keep a hole in the ice. Even the smallest of holes will suffice. This allows oxygen in for the fish and plants and also allows any toxins to escape.
By allowing your pond to completely freeze over during the winter months, the harmful gases produced by fish waste and plant decay cannot escape the ice, becoming deadly to your costly fish and plants.